An organic-inorganic composite resin is a composite resin in which inorganic fine particles at nanometer level are uniformly dispersed in a resin such as a monomer, oligomer, or polymer resin. Those composite resins are more excellent in various physical properties such as optical scattering property, heat resistance, and mechanical property compared with materials in each of which inorganic fine particles at micrometer level are dispersed in a resin.
Particularly in optical fields, optical materials each having a property which can not be achieved by a resin alone or an inorganic substance alone are developed by preparing an optical material using the organic-inorganic composite resin. Examples thereof include an investigation of an optical material in which fine particles of titanium oxide are dispersed in an energy-polymerizable acrylic monomer. Those optical materials each have properties such as a high refraction index, a low Abbe's number (νd), a high secondary dispersion property (θg, F), a high reflectance, a high heat resistance, and a high mechanical strength. From these points, the optical materials are expected to be utilized as useful optical members by molding into a membrane for a hard coating and a shape of a lens.
However, in general, the inorganic fine particles each having a single nanometer diameter size to a several tens nanometer size extremely easily form aggregates in an organic solvent or a monomer, oligomer, or polymer resin. As a result, the scatter occurs and clouding is produced. In particular, when the inorganic fine particles are used for an optical member such as a hard coating and lens, it is necessary to avoid the scatter due to the aggregation. Currently, sols in each of which the fine particles of titanium oxide are dispersed in a solvent are commercially available, but these sols are clouded and are inadequate to use as the optical material required to inhibit the scatter. Further, those sols also lack stability with time.
To uniformly disperse the inorganic fine particles each having the nanometer size in a resin, a method is effective in which first a sol in which the inorganic fine particles are uniformly dispersed in the solvent is prepared and then a resin is dissolved therein. It is also investigated to primarily disperse and stabilize inorganic fine particles by adding a dispersant or a surface treating agent in a solvent or a resin. In particular, when the occurrence of scatter is inhibited, it is necessary to uniformly disperse the inorganic fine particles or the aggregates thereof in a sufficiently smaller state than a wavelength of light.
In such circumstance, PTL 1 describes a composition for coating in which hydrolyzable silane, a titanium compound and/or an organosiloxane oligomer, and metal oxide fine particles are mixed. It is also described that a membrane obtained by applying the composition so that a thickness of the dried membrane be 0.2 μm is excellent in transparency. However, when the solvent is removed without making the composition obtained by the composition into a coated film, clouding occurs sometimes. Further, even when the composition is made into a coated film, clouding occurs when a membrane thickness is 100 μm or more.
Further, PTL 2 describes that a relatively transparent sol is obtained by dispersing titanium oxide in toluene using a saturated fatty acid or an unsaturated fatty acid as a dispersant, pulverizing the resulting sol of titanium oxide using a bead mill, and centrifuging the sol. However, the resultant sol is a slightly clouded liquid with scatter probably because the aggregates can not be removed well.
A solution in which ZEONEX (manufactured by Zeon Corporation) as an organic polymer is dissolved in toluene, which is then mixed with the sol is prepared. A polymer-based nanocomposite is made by removing the toluene solvent from the solution, and it is described that the nanocomposite is also a slightly clouded material with scatter.
So, no organic solvent dispersion and no organic-inorganic composite resin composition in which titanium oxide fine particles are dispersed in a state with extremely low scatter in the solvent without impairing the properties of titanium oxide are substantially obtained.